1. Field of the Invention
The present invention relates to light emission control in an electronic flash device of a camera.
2. Related Background Art
A conventional electronic flash device of a camera uses a light emission control circuit for stopping light emission from the flash device when a light emitted by the flash device and reflected by an object reaches a predetermined light intensity.
The light emission control circuit converts the light emitted from the electronic flash device and reflected by the object to a photo-current by a photosensor, amplifies the converted photo-current by an amplifier and charges up an integration capacitor. When the charge in the integration capacitor reaches a predetermined level (reference voltage), the light emission from the flash device is stopped.
The light emission control of the electronic flash device by such a light emission control circuit can be switched in accordance with a F-value of the camera.
FIG. 1 shows a portion of a light emission control circuit. A reflection light from an object is converted to a photo-current by a photo-sensor 24, and the photo-current is logarithmically amplified by an operational amplifier 22 having a logarithmical compression diode 25 connected in a negative feedback fashion, and an output of the operational amplifier is logarithmically expanded by a transistor 31. An integration capacitor 29 is connected to a collector of the transistor 31 and it is charged up by the photo-current amplified by the transistor 31. A transistor 28 is connected in parallel to the integration capacitor 29 and the charging of the integration capacitor 29 is initiated upon turn-off of the transistor 28 at the start of flashing.
The charged voltage in the capacitor 29 is supplied to a minus input of a comparator 23, and when it reaches a reference voltage of a reference power supply 30 connected to a plus terminal of the comparator, the comparator 23 produces an output to stop the light emission of the flash device.
In such a light emission control circuit, an amplification factor of the amplifier unit comprising the operational amplifier 22 and the transistor 31 is uniquely determined by a difference (Vf-Ve) between a voltage Vf applied to the operational amplifier 22 from a power supply 36 and a voltage Ve applied to an emitter of the transistor 31 through an buffer amplifier 38. The voltage Ve is developed by supplying a current from a constant current cource 27 through resistors 33 and 34 and it is applied through the buffer amplifier 38. Accordingly, a usual way to change the amplification factor is to provide switching means 35a in parallel with the resistor 33 and switch the voltage Ve by turning on and off the switching means 35a. However, the resistor 33 cannot be perfectly shortened even if the switch 35a is turned on because of a contact resistance. Since the resistance of the resistor 33 is small, a precise control of the amplification factor is not attained.
The switching means 35a is usually implemented by a transistor. The voltage difference per stage of F-value is several tens mV while a collector-emitter voltage Vce of the ON transistor is usually approximately 100 mV. As a result, the change of the voltage due to turn-on and turn-off of the transistor is too much to allow the change of the amplification factor in accordance with the selected F-value. In addition, because a mechanical switch is used, a structure of the mechanical switch is complex in order to allow multiple switching of the amplification factor.